Automatic Measurement of Total
Oxygen Demand
DENNIS A. CLIFFORD, Instrument Engineer
Instrument Systems Research Laboratory
Dow Chemical Company
Midland, Michigan
INTRODUCTION
Of the numerous criteria for the determination of water quality, DO is rightfully considered one of the most important since a low level of DO indicates pollution and may mean the suffocation of aquatic life. Also, the "oxygen demand" of a water or water system has long been of interest to us because of its
relationship to DO, and because it is probably the best measure of loading on a
biological waste treatment process.
Admittedly, there is considerable discussion in the literature concerning the
usefulness of the various oxygen demand tests. Nevertheless, the BOD (1) and the
COD (2) have gained nearly universal acceptance as indicators of gross pollution
and loading on oxidation processes.
If properly administered, the BOD test is obviously the better of the two for
approximating what biochemical oxidation will eventually take place in a stream.
Unfortunately, it is a very lengthy process subject to numerous interferences and
special techniques.
Although faster and less subject to interferences than is the BOD test, the COD
method is known not to oxidize such common pollutants as pyridine, benzene, and
ammonia. But for most organic compounds, the oxidation has been reported to be
95-100 per cent of the theoretical. Finally, it is somewhat undesirable in that it
requires the frequent handling of hazardous chemicals.
COD is seldom the same as BOD, nor does it always correlate with BOD. The
usefulness of the COD method is due to the fact that results are obtained in two
hrs rather than five days. For want of a better method, COD has long been an
accepted indicator of water quality.
THE SEARCH FOR BETTER METHODS
Recent emphasis on pollution control of water resources has resulted in a
search for analytical methods that: 1) are meaningful and correlative to the accepted parameters for control and policing; 2) are rapid, so results are known in
mins, not hrs or days; and 3) are truly adaptable to automation and continuous
monitoring.
The Dow Chemical Company has developed several instruments to meet
these needs. They are: The Total Carbon Analyzer (3), The Total Organic Carbon Analyzer (4), The C02D Analyzer (5), and The Streaming Current Detector
(6).
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